1. Field of the Invention
This invention relates generally to a powertrain for a motor vehicle and, more particularly, to controlling torque transmitted between a flywheel and vehicle wheels.
2. Description of the Prior Art
A hybrid vehicle powertrain combines a conventional propulsion system, which includes an internal combustion engine and a step-change automatic transmission, with an energy storage system to improve fuel economy over a conventional vehicle powertrain. Various techniques for regenerating and storing kinetic energy of the vehicle include electric systems, which include an electric motor and electric storage battery; hydraulic systems, which store energy in a pressurized hydraulic tank and flywheel systems, which store energy in a rotating flywheel or disc.
A major challenge present by flywheel systems is the need to provide robustly and smoothly the flywheel torque to the wheels. One way that has been proposed is to determine the wheel torque through the measurement of wheel slip as an input into the control of the flywheel torque delivery to the wheels. This measurement can be difficult since it is very stiff and can change with environmental conditions, creating the need to have very accurate ratio control. Wheel slip is a difficult measurement to make in a vehicle particularly under low loads, and it has a high gain. Small errors in this measurement can cause large control perturbations.